This invention relates to apparatus for use in combination with an operating microscope for the qualitative, semi-quantitative, and quantitative determination of corneal astigmatism of the human eye.
In performing corneal transplants on the human eye, the calotte-shaped transplant of the donor eye is sewn to the adjoining cornea of the receiving eye by very fine suture material having a diameter of about 0.02 millimeters. This delicate operation is possible only with the use of an operating microscope which provides the operator with a three-dimensional true-color image as a magnifying aid in observing the site of the operation.
A difficulty of these operations resides in the fact that, even with extremely careful suturing, the pull exerted by the thread on the transplant is not distributed uniformly. An undesired deformation of the cornea is produced as a result of the differential pull of the thread on the transplant, this undesired deformation being referred to as corneal astigmatism. The corneal astigmatism produced in this way may have a value up to about seven diopters. During the course of healing and cicatrization, the corneal astigmatism which has been unintentionally introduced as a result of the operation can usually be reduced by up to about two diopters, but usually not more than this reduction is possible. Ordinarily the residual astigmatism which can be tolerated is not more than two diopters. If the reduction of the astigmatism during healing can be not more than about two diopters and if the tolerable limit of residual astigmatism is not more than about two diopters, this means that the initial astigmatism unintentionally introduced by the suturing operation should be not more than four diopters, whereas experience shows that it is frequently more than this. Therefore it becomes vitally important to check the astigmatism of the cornea at the very moment that the suturing is being accomplished, in order to make sure that the suturing does not introduce too great an amount of astigmatism, and to be able to take immediate corrective action if it is found that too great an amount is introduced. The present invention provides simple and effective means for accomplishing this checking of corneal astigmatism at the time that the transplant operation is in progress.